Despite the increasingly frequent use of renewable energies to produce electricity avoiding concomitant production of CO2, it is reasonable to consider that CO2 emissions, in particular resulting from energy production, will remain high in the next decades. It thus appears necessary to find ways to capture CO2 gas, either for storing or valorization purposes.
Indeed, CO2 can also be seen, not as a waste, but on the contrary as a source of carbon. For example the promising production of synthetic fuels from CO2 and water has been envisaged.
However, CO2 exhibits low chemical reactivity: breaking its bonds requires an energy of 724 kJ/mol. Moreover, CO2 electrochemical reduction to one electron occurs at a very negative potential, thus necessitating a high energy input, and leads to the formation of a highly energetic radical anion (CO2.−); catalysis thus appears mandatory in order to reduce CO2 and drive the process to multi-electronic and multi-proton reduction process, in order to obtain thermodynamically stable molecules. In addition, direct electrochemical reduction of CO2 at inert electrodes is poorly selective, yielding to formic acid in water, while it yields a mixture of oxalate, formate and carbon monoxide in low-acidity solvents such as DMF.
Electrolysis is a method of applying a potential at an immersed electrode to drive an otherwise non-spontaneous electrochemical reaction. Electrolysis is performed in an electrochemical cell, comprising at least:                an electrolyte solution comprising the solvent, a supporting electrolyte as a salt, and the substrate        a power supply providing the energy necessary to trigger the electrochemical reactions involving the substrate; and        two electrodes, i.e. electrical conductors providing a physical interface between the electrical circuit and the solution.        
CO2 electrochemical reduction requires catalytic activation in order to reduce the energy cost of processing, and increase the selectivity of the species formed in the reaction process.
Such systems require a complex molecular machinery and only a few homogeneous catalysts have been described to date, and they are almost exclusively based on quite expensive rare metals.
Homogeneous or heterogeneous catalysts based on transition metals of the first line (Mn, Fe, Co, Ni, Cu), which appear preferable because of their availability and low cost, are also used for the reduction of CO2. However, whether these metals are used in the form of a complex, as e.g. complexes of porphyrin, phthalocyanine, polypyridine, or cyclam, the resulting catalysts are less efficient than their counterparts based on transition metals of the second and third lines (Ru, Rh, Pd, Re, Pt, . . . ) (for a review see Savéant Chem. Rev. 2008, 108, 2348-2378).
In particular, iron porphyrins have been previously described, but their catalytic properties regarding the electrochemical reduction of CO2 into CO were rather poor (see for instance JP 2003-260364 and WO 2011/150422). Bhugun et al (see in particular J. Am. Chem. Soc. 1994, 116, 5015-5016 and J. Am. Chem. Soc. 1996, 118, 1769-1776) however demonstrated that the selectivity and TON (see definition below) of the iron porphyrin catalysts, such as in particular Fe-TPP (5,10,15,20-tetrakisphenylporphyrine), are significantly increased when adding either a Lewis acid or a Brönsted acid to the electrolyte solution. Said acid indeed acts as a synergistic factor with the catalyst. However, the mechanism of action of said acid remains to be precisely determined. Moreover, when the acid strength increases, it may result in a loss of selectivity and a progressive deterioration of the catalyst.
There thus remains a strong need for catalysts for the electrochemical reduction of CO2 into CO based on iron porphyrin with high efficiency (i.e. high faradic yield, high Turnover Number (TON) and Turnover Frequency (TOF)), high selectivity and high stability, while if possible operating at a low overpotential.